Many medical devices incorporate elongate shafts such as tubes which are intended for insertion into and through passageways of a living body such as those of the urethral tract and the cardiovascular system. The most common type of this general grouping of medical devices is known as catheters. Exemplary catheters include those designated for urological, angioplasty and valvuloplasty uses, that is, adapted respectively for insertion into the urethra, the lumen of a blood vessel and heart passageway of a living body, normally a human body.
Because of the intended use of such medical devices certain parameters need to be satisfied by the material from which the elongate shaft is manufactured. The material must fulfill such requirements as softness, good kink resistance, good dimensional stability, processability, for example ease to form and glue, and the possibility to be sterilized by radiation, steam, ethylene oxide or other means. For some products, there is further the need for the material to accept a surface treatment which will impart desired surface properties to the medical device, such as hydrophilicity. To this latter end, the chemistry of the substrate material is critical since this affects the possibility to coat the substrate.
For many years now polyvinyl chloride (PVC) has been used to manufacture medical devices having elongate shafts for insertion into a body passageway such as catheters due to PVC fulfilling the requirements mentioned in the preceding paragraph. For instance, EP 0 093 093 by the same applicant makes known a process for manufacturing a PVC urinary catheter having a hydrophilic outer surface coating which exhibits a low coefficient of friction when wetted.
However, the suitability of PVC for medical devices such as catheters is now being questioned on environmental grounds and further because of the toxicity of the plasticizers added to PVC. Moreover, coating PVC catheters by, for example, the EP 0 093 093 results in an appreciable shrinkage of the PVC catheters in the longitudinal direction, typically 6-7% of the original length, due to the operating temperatures used in the coating process. The obvious disadvantage of such appreciable shrinkage is the wastage of material in the sense that PVC catheters of longer length than finally required have to be used to account for the shrinkage. In addition, quality control of the coating process is made more complicated than would be ideal by this marked degree of shrinkage.
Other substrate materials have also been proposed. For example, WO 97/49437 by the same applicant proposes to use a polyether block amide and a styrene block copolymer as substrate material for a hydrophilic catheter. These materials have proven to be suitable for hydrophilic coating, and to have adequate mechanical and chemical properties. However, a problem with these materials is that these materials are relatively expensive to manufacture. Further, polyether block amide has relatively high resilience, which makes it unsuitable for certain applications. For example, catheters made of this material may be difficult to handle for disabled patients. When using styrene block copolymer, the adherence of surface coatings, such as hydrophilic coatings, is lower than when using e.g. polyether block amide.
Thus, there is a general problem for most previously known catheter substrates, that they are costly and/or harmful to the environment, and/or that there are problems related to the hydrophilic coating, such as too poor water retention properties, especially after leaching, too poor adherence to the substrate and too high friction of the hydrophilic surface when wetted. Further, alternatively or additionally, the mechanical properties of the substrates may be inadequate, such as being too stiff or having too high resilience.
There is therefore a need for a new substrate material for medical devices to be coated with a hydrophilic surface coating, which is environmentally acceptable and cost effective, to which the hydrophilic coating can be adequately adhered, and which has adequate mechanical and chemical properties.